Instruments are utilized in testing the performance of combustion engines for automobiles and these instruments include gauges for measuring inlet manifold vacuum, fuel pump pressure and cylinder compression. Generally, separate instrument are utilized for each such measurement. However, in the case of inlet manifold vacuum or fuel pump pressure, a bourdon type gauge has been utilized where the vacuum and pressure are connected to opposite ends of the bourdon tube and the zero indicator arm is at the center of the scale. When pressure is connected with the gauge, the indicator arm reads on one half of the scale and when vacuum is attached to the gauge, the indicator arm reads on the other half of the scale. Thus, only one half of the scale is available for obtaining either measurement.
Separate pressure gauges have also been used for the measurement of fuel pump pressure and the measurement of cylinder compression pressure because of the large difference in magnitude of these pressures. While the gauges for measuring pressures usually consist of a piston working against a spring, a larger spring has to be used for the measurement of compression pressure and therefore, instrument casings of different sizes and design have been required for each of these types of pressure gauges.